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        </div><article class="page single"><h1 class="single-title animated flipInX">Spring源码 - 循环依赖的实现原理</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="https://wlizhi.cc" title="Author" target="_blank" rel="noopener noreffer author" class="author"><i class="fas fa-user-circle fa-fw"></i>Wlizhi</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/spring/"><i class="far fa-folder fa-fw"></i>spring</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw"></i>&nbsp;<time datetime="2020-11-27">2020-11-27</time>&nbsp;<i class="fas fa-pencil-alt fa-fw"></i>&nbsp;约 4547 字&nbsp;
                <i class="far fa-clock fa-fw"></i>&nbsp;预计阅读 10 分钟&nbsp;<span id="/posts/spring/06-cyclic-dependence/" class="leancloud_visitors" data-flag-title="Spring源码 - 循环依赖的实现原理">
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                <div class="details-content toc-content" id="toc-content-static"><nav id="TableOfContents">
  <ul>
    <li><a href="#1-什么是循环依赖">1 什么是循环依赖</a></li>
    <li><a href="#2-循环依赖在spring中的实现原理">2 循环依赖在spring中的实现原理</a>
      <ul>
        <li><a href="#21-源码分析">2.1 源码分析</a></li>
        <li><a href="#22-源码中要注意的细节">2.2 源码中要注意的细节</a></li>
        <li><a href="#23-流程梳理">2.3 流程梳理</a></li>
      </ul>
    </li>
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            </div><div class="content" id="content"><p>spring源码系列文章，示例代码的中文注释，均是 copy 自 <a href="https://gitee.com/wlizhi/spring-framework" target="_blank" rel="noopener noreffer">https://gitee.com/wlizhi/spring-framework</a> 。</p>
<p>链接中源码是作者从 github 下载，并以自身理解对核心流程及主要节点做了详细的中文注释。</p>
<hr>
<h2 id="1-什么是循环依赖">1 什么是循环依赖</h2>
<p>在我们日常开发中，肯定存在这种情况：bean A 某个成员是 bean B，bean B 中某个属性是 bean A。那A类和B类就是相互依赖的关系，也叫循环依赖。</p>
<p>当然，也可以是这种情况 A -&gt; B -&gt; C -&gt; D -&gt; E -&gt; A。这种关系也是循环依赖。</p>
<h2 id="2-循环依赖在spring中的实现原理">2 循环依赖在spring中的实现原理</h2>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="nd">@Service</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CyclicGoodsServiceImpl</span> <span class="kd">implements</span> <span class="n">CyclicGoodsService</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Autowired</span>
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="n">CyclicOrderService</span> <span class="n">cyclicOrderService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">CyclicOrderService</span> <span class="nf">getCyclicOrderService</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">cyclicOrderService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
<h3 id="21-源码分析">2.1 源码分析</h3>
<p><span id='ciclic-demo'>在这之前，先看一个简单的循环依赖代码案例</span>：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="nd">@Service</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CyclicGoodsServiceImpl</span> <span class="kd">implements</span> <span class="n">CyclicGoodsService</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">	<span class="nd">@Autowired</span>
</span></span><span class="line hl"><span class="cl">	<span class="kd">private</span> <span class="n">CyclicOrderService</span> <span class="n">cyclicOrderService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">CyclicOrderService</span> <span class="nf">getCyclicOrderService</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">cyclicOrderService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="nd">@Service</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">CyclicOrderServiceImpl</span> <span class="kd">implements</span> <span class="n">CyclicOrderService</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">	<span class="nd">@Autowired</span>
</span></span><span class="line hl"><span class="cl">	<span class="kd">private</span> <span class="n">CyclicGoodsService</span> <span class="n">cyclicGoodsService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">CyclicGoodsService</span> <span class="nf">getCyclicGoodsService</span><span class="o">()</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">cyclicGoodsService</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="nd">@Slf4j</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">ApplicationContextTest</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="nd">@Test</span>
</span></span><span class="line"><span class="cl">    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">testCyclicDependence</span><span class="o">(){</span>
</span></span><span class="line"><span class="cl">        <span class="n">AnnotationConfigApplicationContext</span> <span class="n">context</span> <span class="o">=</span> <span class="k">new</span> <span class="n">AnnotationConfigApplicationContext</span><span class="o">(</span><span class="s">&#34;top.wlz922.cyclic&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="n">CyclicGoodsService</span> <span class="n">goodsService</span> <span class="o">=</span> <span class="n">context</span><span class="o">.</span><span class="na">getBean</span><span class="o">(</span><span class="n">CyclicGoodsService</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="n">CyclicOrderService</span> <span class="n">orderService</span> <span class="o">=</span> <span class="n">context</span><span class="o">.</span><span class="na">getBean</span><span class="o">(</span><span class="n">CyclicOrderService</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="n">goodsService</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">        <span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="n">orderService</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
只是简单的打印一下两个Service类中属性的值，很明显是注入成功了。</p>
<p>以下是打印的内容：</p>
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<pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback"><span class="line"><span class="cl">19:34:42.334 [main] DEBUG top.wlz922.test.context.ApplicationContextTest 36 - top.wlz922.cyclic.CyclicGoodsServiceImpl@7fa98a66
</span></span><span class="line"><span class="cl">19:34:42.335 [main] DEBUG top.wlz922.test.context.ApplicationContextTest 37 - top.wlz922.cyclic.CyclicOrderServiceImpl@15ff3e9e
</span></span></code></pre></td></tr></table>
</div>
</div><hr>
<p>前面的文章中已经分析过，spring中获取bean实例的方法是getBean()。在容器启动的时候，会调用这些方法，实例化单例bean实例。</p>
<p>直接来到doGetBean()，看以下代码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractBeanFactory</span> <span class="kd">extends</span> <span class="n">FactoryBeanRegistrySupport</span> <span class="kd">implements</span> <span class="n">ConfigurableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">T</span> <span class="nf">doGetBean</span><span class="o">(</span><span class="kd">final</span> <span class="n">String</span> <span class="n">name</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="kd">final</span> <span class="n">Class</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">requiredType</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">	                          <span class="nd">@Nullable</span> <span class="kd">final</span> <span class="n">Object</span><span class="o">[]</span> <span class="n">args</span><span class="o">,</span> <span class="kt">boolean</span> <span class="n">typeCheckOnly</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">BeansException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="c1">//转换beanName，这里传入进来的beanName可能是以&amp;开头的，这里会截掉&amp;
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="kd">final</span> <span class="n">String</span> <span class="n">beanName</span> <span class="o">=</span> <span class="n">transformedBeanName</span><span class="o">(</span><span class="n">name</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="n">Object</span> <span class="n">bean</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="cm">/*
</span></span></span><span class="line"><span class="cl"><span class="cm">		TODO 这里多级缓存用于解决循环依赖问题。
</span></span></span><span class="line"><span class="cl"><span class="cm">			注：循环依赖只能是属性见的依赖，不能是构造函数中参数的循环依赖。
</span></span></span><span class="line"><span class="cl"><span class="cm">		    从缓存（一级二级三级缓存依次获取，获取到就返回）中获取单例实例，如果获取到并且是无参构造函数，则将bean变量的值赋值。
</span></span></span><span class="line"><span class="cl"><span class="cm">		 */</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// Eagerly check singleton cache for manually registered singletons.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="n">Object</span> <span class="n">sharedInstance</span> <span class="o">=</span> <span class="n">getSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">sharedInstance</span> <span class="o">!=</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">args</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 省略无关代码
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="c1">// 如果bean不是FactoryBean类型或者beanName以&amp;开头，则直接返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="c1">// 否则将bean强转为FactoryBean类型，并调用getObject()方法返回。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">bean</span> <span class="o">=</span> <span class="n">getObjectForBeanInstance</span><span class="o">(</span><span class="n">sharedInstance</span><span class="o">,</span> <span class="n">name</span><span class="o">,</span> <span class="n">beanName</span><span class="o">,</span> <span class="kc">null</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span> 
</span></span><span class="line"><span class="cl">		<span class="c1">// 省略无关代码...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">return</span> <span class="o">(</span><span class="n">T</span><span class="o">)</span> <span class="n">bean</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p><span id="get-singleton-source"></span>
先跳过无关代码，看关键点。从上面代码中可以看出，在doGetBean()中先调用了getSingleton()，如果这个值不为空就调用getObjectForBeanInstance()，经过一定的处理（先不要关心具体处理了什么），返回这个bean实例。</p>
<p>可以先分析，从接收变量的变量名sharedInstance就可以看出来，这是一个bean实例。关键点就在getSingleton()方法，这里可能会获取到一个bean实例。</p>
<p>下面是 getSingleton() 源码：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">DefaultSingletonBeanRegistry</span> <span class="kd">extends</span> <span class="n">SimpleAliasRegistry</span> <span class="kd">implements</span> <span class="n">SingletonBeanRegistry</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="cm">/** 一级缓存 里面存储的是bean实例，在bean实例化完成、依赖注入、代理生成等流程全部完成之后，会存储到这个容器中 */</span>
</span></span><span class="line"><span class="cl">    <span class="cm">/** Cache of singleton objects: bean name to bean instance. */</span>
</span></span><span class="line"><span class="cl">    <span class="kd">private</span> <span class="kd">final</span> <span class="n">Map</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">Object</span><span class="o">&gt;</span> <span class="n">singletonObjects</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ConcurrentHashMap</span><span class="o">&lt;&gt;(</span><span class="n">256</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    
</span></span><span class="line"><span class="cl">    <span class="cm">/** 
</span></span></span><span class="line"><span class="cl"><span class="cm">     * 三级缓存，在单例bean刚刚创建完成时，会将其封装成一个ObjectFactory对象，存储到这个容器中。键值是beanName。
</span></span></span><span class="line"><span class="cl"><span class="cm">     * 从源码中可以看到，实际上是提供了一些通过BeanPostProcessor进行的扩展操作。这里不会执行，只是把这个扩展操作流程封装起来。
</span></span></span><span class="line"><span class="cl"><span class="cm">     * 实际上，这里封装的一个函数式接口，在这里可以通过BeanPostProcessor进行一些扩展操作，其节点是bean实例化之后，
</span></span></span><span class="line"><span class="cl"><span class="cm">     * 在真正getBean时，会触发调用。 */</span>
</span></span><span class="line"><span class="cl">    <span class="cm">/** Cache of singleton factories: bean name to ObjectFactory. */</span>
</span></span><span class="line"><span class="cl">    <span class="kd">private</span> <span class="kd">final</span> <span class="n">Map</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">ObjectFactory</span><span class="o">&lt;?&gt;&gt;</span> <span class="n">singletonFactories</span> <span class="o">=</span> <span class="k">new</span> <span class="n">HashMap</span><span class="o">&lt;&gt;(</span><span class="n">16</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="cm">/** 
</span></span></span><span class="line"><span class="cl"><span class="cm">     * 二级缓存 从三级缓存获取后，会将bean实例存储到二级缓存，并从三级缓存中移除。这里封装了一个函数式接口的匿名实例，
</span></span></span><span class="line"><span class="cl"><span class="cm">     *  依赖注入过程中通过getBean方法获取这个实例时，可以对其进行一些指定操作，比如说aop代理对象生成，就可能在这里触发。
</span></span></span><span class="line"><span class="cl"><span class="cm">     */</span>
</span></span><span class="line"><span class="cl">    <span class="cm">/** Cache of early singleton objects: bean name to bean instance. */</span>
</span></span><span class="line"><span class="cl">    <span class="kd">private</span> <span class="kd">final</span> <span class="n">Map</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">Object</span><span class="o">&gt;</span> <span class="n">earlySingletonObjects</span> <span class="o">=</span> <span class="k">new</span> <span class="n">HashMap</span><span class="o">&lt;&gt;(</span><span class="n">16</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getSingleton</span><span class="o">(</span><span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="kt">boolean</span> <span class="n">allowEarlyReference</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="cm">/**
</span></span></span><span class="line"><span class="cl"><span class="cm">		 * 从一级缓存中取，如果有值，则返回。否则依次查找二级、三级缓存，如果最终从三级缓存中拿到值，
</span></span></span><span class="line"><span class="cl"><span class="cm">		 * 则将bean对象升级到二级缓存，并把原值从三级缓存中移除。
</span></span></span><span class="line"><span class="cl"><span class="cm"> 		 */</span>
</span></span><span class="line hl"><span class="cl">		<span class="n">Object</span> <span class="n">singletonObject</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">singletonObject</span> <span class="o">==</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">isSingletonCurrentlyInCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="kd">synchronized</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">				<span class="n">singletonObject</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">earlySingletonObjects</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="k">if</span> <span class="o">(</span><span class="n">singletonObject</span> <span class="o">==</span> <span class="kc">null</span> <span class="o">&amp;&amp;</span> <span class="n">allowEarlyReference</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">					<span class="n">ObjectFactory</span><span class="o">&lt;?&gt;</span> <span class="n">singletonFactory</span> <span class="o">=</span> <span class="k">this</span><span class="o">.</span><span class="na">singletonFactories</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="k">if</span> <span class="o">(</span><span class="n">singletonFactory</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">singletonObject</span> <span class="o">=</span> <span class="n">singletonFactory</span><span class="o">.</span><span class="na">getObject</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">						<span class="k">this</span><span class="o">.</span><span class="na">earlySingletonObjects</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">singletonObject</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="k">this</span><span class="o">.</span><span class="na">singletonFactories</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">singletonObject</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
从源码添加的中文注释中可以看出，BeanFactory有这么三个关键成员。分别是一、二、三级缓存。</p>
<blockquote>
<p>一级缓存 singletonObjects：封装了最终完成了所有处理流程的Bean。<br>
二级缓存 earlySingletonObjects：封装了通过ObjectFactory.getObject()处理过后的对象。<br>
三级缓存 singletonFactories：Bean刚刚创建完实例后，将Bean实例封装成ObjectFactory对象，存储到这里。</p>
</blockquote>
<p>暂且记着这三个缓存的作用，来找bean被存储到三级缓存的源头。</p>
<p>在一个bean刚刚创建的时候，会调用到doCreateBean方法，在这个方法中真正实例化了bean，那么实例化完成之后，紧接着就会将其包装成ObjectFactory对象，存储到三级缓存。来到这块代码：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractAutowireCapableBeanFactory</span> <span class="kd">extends</span> <span class="n">AbstractBeanFactory</span>
</span></span><span class="line"><span class="cl">		<span class="kd">implements</span> <span class="n">AutowireCapableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="kd">protected</span> <span class="n">Object</span> <span class="nf">doCreateBean</span><span class="o">(</span><span class="kd">final</span> <span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="kd">final</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span><span class="o">,</span> <span class="kd">final</span> <span class="nd">@Nullable</span> <span class="n">Object</span><span class="o">[]</span> <span class="n">args</span><span class="o">)</span>
</span></span><span class="line"><span class="cl">    			<span class="kd">throws</span> <span class="n">BeanCreationException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 省略无关代码...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// 创建bean实例
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="k">if</span> <span class="o">(</span><span class="n">instanceWrapper</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">            <span class="n">instanceWrapper</span> <span class="o">=</span> <span class="n">createBeanInstance</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">mbd</span><span class="o">,</span> <span class="n">args</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">        <span class="kd">final</span> <span class="n">Object</span> <span class="n">bean</span> <span class="o">=</span> <span class="n">instanceWrapper</span><span class="o">.</span><span class="na">getWrappedInstance</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// 省略代码：对一些注解支持的扫描、合并到BeanDefinition...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// Eagerly cache singletons to be able to resolve circular references
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// even when triggered by lifecycle interfaces like BeanFactoryAware.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="kt">boolean</span> <span class="n">earlySingletonExposure</span> <span class="o">=</span> <span class="o">(</span><span class="n">mbd</span><span class="o">.</span><span class="na">isSingleton</span><span class="o">()</span> <span class="o">&amp;&amp;</span> <span class="k">this</span><span class="o">.</span><span class="na">allowCircularReferences</span> <span class="o">&amp;&amp;</span>
</span></span><span class="line"><span class="cl">                <span class="n">isSingletonCurrentlyInCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">));</span>
</span></span><span class="line"><span class="cl">        <span class="k">if</span> <span class="o">(</span><span class="n">earlySingletonExposure</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">if</span> <span class="o">(</span><span class="n">logger</span><span class="o">.</span><span class="na">isTraceEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">                <span class="n">logger</span><span class="o">.</span><span class="na">trace</span><span class="o">(</span><span class="s">&#34;Eagerly caching bean &#39;&#34;</span> <span class="o">+</span> <span class="n">beanName</span> <span class="o">+</span>
</span></span><span class="line"><span class="cl">                        <span class="s">&#34;&#39; to allow for resolving potential circular references&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">            <span class="o">}</span>
</span></span><span class="line"><span class="cl">            <span class="c1">// 提前暴露bean实例的引用，这时候的bean是刚刚创建完成的，还没有进行属性的依赖注入、等流程处理。
</span></span></span><span class="line hl"><span class="cl"><span class="c1"></span>            <span class="n">addSingletonFactory</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="o">()</span> <span class="o">-&gt;</span> <span class="n">getEarlyBeanReference</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">mbd</span><span class="o">,</span> <span class="n">bean</span><span class="o">));</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
从上面列出的spring源码中可以看到，当一个bean创建完实例后，会调用addSingletonFactory()，这个方法的第二个参数，传递了一个匿名对象，这个对象就是ObjectFactory类型的。</p>
<p>来到addSingletonFactory()源码：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">DefaultSingletonBeanRegistry</span> <span class="kd">extends</span> <span class="n">SimpleAliasRegistry</span> <span class="kd">implements</span> <span class="n">SingletonBeanRegistry</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="kt">void</span> <span class="nf">addSingletonFactory</span><span class="o">(</span><span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">ObjectFactory</span><span class="o">&lt;?&gt;</span> <span class="n">singletonFactory</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">notNull</span><span class="o">(</span><span class="n">singletonFactory</span><span class="o">,</span> <span class="s">&#34;Singleton factory must not be null&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="kd">synchronized</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">if</span> <span class="o">(!</span><span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">.</span><span class="na">containsKey</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 将这个bean的匿名对象，存储到三级缓存。匿名对象中封装了一个扩展点的操作。
</span></span></span><span class="line hl"><span class="cl"><span class="c1"></span>				<span class="k">this</span><span class="o">.</span><span class="na">singletonFactories</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">singletonFactory</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="k">this</span><span class="o">.</span><span class="na">earlySingletonObjects</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="k">this</span><span class="o">.</span><span class="na">registeredSingletons</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
可以看到，在bean刚刚创建完成，就把其封装到了ObjectFactory中，存储到了三级缓存singletonFactories（一个Map）。</p>
<p>从调用这个方法的地方（addSingletonFactory()），可以看到，这个匿名函数的内容：() -&gt; getEarlyBeanReference(beanName, mbd, bean)</p>
<p>来看一下这个匿名对象具体做了什么：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractAutowireCapableBeanFactory</span> <span class="kd">extends</span> <span class="n">AbstractBeanFactory</span>
</span></span><span class="line"><span class="cl">		<span class="kd">implements</span> <span class="n">AutowireCapableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="n">Object</span> <span class="nf">getEarlyBeanReference</span><span class="o">(</span><span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">RootBeanDefinition</span> <span class="n">mbd</span><span class="o">,</span> <span class="n">Object</span> <span class="n">bean</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">Object</span> <span class="n">exposedObject</span> <span class="o">=</span> <span class="n">bean</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 获取早期的Bean对象的引用，这时候的Bean还没有完成依赖注入及其后面的流程。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// 在这里获取引用时，支持一些扩展操作，其扩展内容是通过 getEarlyBeanReference() 实现的。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="c1">// getEarlyBeanReference() 来自 BeanPostProcessor 的子接口 SmartInstantiationAwareBeanPostProcessor。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">if</span> <span class="o">(!</span><span class="n">mbd</span><span class="o">.</span><span class="na">isSynthetic</span><span class="o">()</span> <span class="o">&amp;&amp;</span> <span class="n">hasInstantiationAwareBeanPostProcessors</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">			<span class="k">for</span> <span class="o">(</span><span class="n">BeanPostProcessor</span> <span class="n">bp</span> <span class="o">:</span> <span class="n">getBeanPostProcessors</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="k">if</span> <span class="o">(</span><span class="n">bp</span> <span class="k">instanceof</span> <span class="n">SmartInstantiationAwareBeanPostProcessor</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="n">SmartInstantiationAwareBeanPostProcessor</span> <span class="n">ibp</span> <span class="o">=</span> <span class="o">(</span><span class="n">SmartInstantiationAwareBeanPostProcessor</span><span class="o">)</span> <span class="n">bp</span><span class="o">;</span>
</span></span><span class="line hl"><span class="cl">					<span class="n">exposedObject</span> <span class="o">=</span> <span class="n">ibp</span><span class="o">.</span><span class="na">getEarlyBeanReference</span><span class="o">(</span><span class="n">exposedObject</span><span class="o">,</span> <span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">exposedObject</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
从源码中可以看出：在返回早期暴露的Bean实例引用之前，会通过 BeanPostProcessor 进行一些处理。</p>
<p>处理内容：
<div class="details admonition abstract open">
        <div class="details-summary admonition-title">
            <i class="icon fas fa-list-ul fa-fw"></i>摘要<i class="details-icon fas fa-angle-right fa-fw"></i>
        </div>
        <div class="details-content">
            <div class="admonition-content"><ol>
<li>获取到容器中所有已注册的BeanPostProcessor。</li>
<li>判断是否是其子接口SmartInstantiationAwareBeanPostProcessor类型，如果是，强转。</li>
<li>调用 getEarlyBeanReference() 获取Bean提前暴露对象的引用，在这里可以对Bean实例做一些扩展操作（比如生成代理对象等）。</li>
</ol>
</div>
        </div>
    </div></p>
<p>在addSingletonFactory()，可以看到，这里只是将扩展操作封装起来，放入三级缓存，并没有调用它。</p>
<p>回到文章开头部分，依次从一、二、三级缓存中获取实例，在执行三级缓存获取操作时，就会调用这个方法，拿到Bean提前暴露的引用。拿到之后从三级缓存中移除，放入二级缓存。然后返回。</p>
<p>这也解释了为什么多个实例之间相互依赖，spring依然能够将依赖实例注入到对应成员中。</p>
<p>还有一个点，在哪里放入到一级缓存的？前面的文章中，有列出创建bean实例的入口，createBean方法是在一个函数式接口方法中调用的，它的上层方法是getSingleton()。</p>
<p>来到getSingleton()源码：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">DefaultSingletonBeanRegistry</span> <span class="kd">extends</span> <span class="n">SimpleAliasRegistry</span> <span class="kd">implements</span> <span class="n">SingletonBeanRegistry</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">Object</span> <span class="nf">getSingleton</span><span class="o">(</span><span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">ObjectFactory</span><span class="o">&lt;?&gt;</span> <span class="n">singletonFactory</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">Assert</span><span class="o">.</span><span class="na">notNull</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="s">&#34;Bean name must not be null&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="kd">synchronized</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 省略无关代码...	
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>            <span class="c1">//这里调用了外层传进来的匿名内部类，lambda表达式。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>            <span class="n">singletonObject</span> <span class="o">=</span> <span class="n">singletonFactory</span><span class="o">.</span><span class="na">getObject</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">            <span class="n">newSingleton</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">				
</span></span><span class="line"><span class="cl">            <span class="c1">// 省略无关代码...
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>            <span class="c1">// 加入一级缓存
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>            <span class="k">if</span> <span class="o">(</span><span class="n">newSingleton</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line hl"><span class="cl">                <span class="n">addSingleton</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">singletonObject</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">            <span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">singletonObject</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
singletonFactory.getObject() 中调用了createBean，这里完成了bean的实例化、依赖注入、代理生成等等操作。</p>
<p>在createBean所有操作完成之后，最后调用了addSingleton()方法。这里把bean实例加入到了一级缓存，并从二级缓存中移除。</p>
<p>来到addSingleton()源码：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">DefaultSingletonBeanRegistry</span> <span class="kd">extends</span> <span class="n">SimpleAliasRegistry</span> <span class="kd">implements</span> <span class="n">SingletonBeanRegistry</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="kt">void</span> <span class="nf">addSingleton</span><span class="o">(</span><span class="n">String</span> <span class="n">beanName</span><span class="o">,</span> <span class="n">Object</span> <span class="n">singletonObject</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="kd">synchronized</span> <span class="o">(</span><span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">this</span><span class="o">.</span><span class="na">singletonObjects</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="n">beanName</span><span class="o">,</span> <span class="n">singletonObject</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="k">this</span><span class="o">.</span><span class="na">singletonFactories</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="k">this</span><span class="o">.</span><span class="na">earlySingletonObjects</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="k">this</span><span class="o">.</span><span class="na">registeredSingletons</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div>
很简单的逻辑，只是把一个对象放入到一级缓存，并从二级、三级缓存中移除。最后一个是已注册的单例bean容器，暂不讨论。</p>
<h3 id="22-源码中要注意的细节">2.2 源码中要注意的细节</h3>
<div class="details admonition note open">
        <div class="details-summary admonition-title">
            <i class="icon fas fa-pencil-alt fa-fw"></i>注意<i class="details-icon fas fa-angle-right fa-fw"></i>
        </div>
        <div class="details-content">
            <div class="admonition-content"><p>构造函数中的依赖注入不可以循环依赖，多例依赖注入也不可以循环依赖。</p>
<ul>
<li>
<p>构造函数不允许循环依赖：很容易明白，如果想要创建一个对象，比如调用它的构造函数，即使反射调用也是如此。如果A构造函数依赖B，B构造函数中依赖A，那么构造函数就无法调用完成，对象也就无从创建。永远也拿不到提前暴露的对象引用。</p>
</li>
<li>
<p>多例情况：不允许循环依赖，由于是多例，每次getBean都会创建新的实例，假如代码中定义了有循环依赖的属性，如果被允许，就会陷入死循环。</p>
</li>
</ul>
</div>
        </div>
    </div>
<p>多例情况，如果有循环依赖，会抛出异常，源码如下：
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">abstract</span> <span class="kd">class</span> <span class="nc">AbstractBeanFactory</span> <span class="kd">extends</span> <span class="n">FactoryBeanRegistrySupport</span> <span class="kd">implements</span> <span class="n">ConfigurableBeanFactory</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">protected</span> <span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">T</span> <span class="nf">doGetBean</span><span class="o">(</span><span class="kd">final</span> <span class="n">String</span> <span class="n">name</span><span class="o">,</span> <span class="nd">@Nullable</span> <span class="kd">final</span> <span class="n">Class</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="n">requiredType</span><span class="o">,</span>
</span></span><span class="line"><span class="cl">                                  <span class="nd">@Nullable</span> <span class="kd">final</span> <span class="n">Object</span><span class="o">[]</span> <span class="n">args</span><span class="o">,</span> <span class="kt">boolean</span> <span class="n">typeCheckOnly</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">BeansException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">        <span class="c1">// Fail if we&#39;re already creating this bean instance:
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// We&#39;re assumably within a circular reference.
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="c1">// 多例不支持循环引用。多例时，如果返回指定的原型bean是否正在创建中，则抛出异常。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>        <span class="k">if</span> <span class="o">(</span><span class="n">isPrototypeCurrentlyInCreation</span><span class="o">(</span><span class="n">beanName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">throw</span> <span class="k">new</span> <span class="n">BeanCurrentlyInCreationException</span><span class="o">(</span><span class="n">beanName</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">        <span class="o">}</span>
</span></span><span class="line"><span class="cl">    <span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div></p>
<p>构造函数中如果有循环依赖，源码ConstructorResolver类中有这段代码，抛出异常：
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span><span class="lnt">4
</span><span class="lnt">5
</span><span class="lnt">6
</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="c1">// 省略无关代码，如果这里有构造函数中的循环依赖，则抛出异常
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="o">}</span><span class="k">catch</span> <span class="o">(</span><span class="n">BeansException</span> <span class="n">ex</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">    <span class="k">throw</span> <span class="k">new</span> <span class="n">UnsatisfiedDependencyException</span><span class="o">(</span>
</span></span><span class="line"><span class="cl">            <span class="n">mbd</span><span class="o">.</span><span class="na">getResourceDescription</span><span class="o">(),</span> <span class="n">beanName</span><span class="o">,</span> <span class="k">new</span> <span class="n">InjectionPoint</span><span class="o">(</span><span class="n">methodParam</span><span class="o">),</span> <span class="n">ex</span><span class="o">);</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div></p>
<h3 id="23-流程梳理">2.3 流程梳理</h3>
<p>例如有A、B两个类进行实例化，分别将对方的实例注入到自身成员中，代码如下（<a href="#ciclic-demo" rel="">前文中类似案例</a>）：
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre tabindex="0" class="chroma"><code><span class="lnt"> 1
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</span><span class="lnt"> 9
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</span></code></pre></td>
<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="nd">@Service</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">A</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Autowired</span>
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="n">B</span> <span class="n">b</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="nd">@Service</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">B</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">	<span class="nd">@Autowired</span>
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="n">A</span> <span class="n">a</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span></span></span></code></pre></td></tr></table>
</div>
</div></p>
<p>分析上面代码的执行流程：</p>
<ol>
<li>A 类无参构造函数实例化后， 设置三级缓存。</li>
<li>A 类 populateBean 进行依赖注入， 这里触发了属性 B 的 getBean 操作。</li>
<li>B 类无参构造函数实例化后，设置三级缓存。</li>
<li>B 类 populateBean 进行依赖注入，这里触发了属性 A 的 getBean 操作</li>
<li>A 类之前正在实例化， singletonsCurrentlyInCreation 集合中有已经有这个 A 实例了，三级缓存里面也有了，所以这时候是从三级缓存中拿到的提前暴露的 A 实例，该实例还没有进行属性 B 的依赖注入，属性 B 为空。</li>
<li>B 类拿到了 A 的提前暴露实例注入到属性 A 中了。</li>
<li>B 类实例化已经完成，B 类的实例化是由 A 类实例化中属性 B 的依赖注入触发的 getBean 操作进行的，现在 B 已经实例化，所以 A 类中属性 B 就可以完成依赖注入了，这时候 A 类 B 属性已经有值了。</li>
<li>B 类 A 属性指向的就是 A 类实例堆空间，所以这时候 B 类 A 属性也会有值了。</li>
</ol></div>

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